A color image sensor typically produces three primary color images. The three primary colors typically are red (R), green (G), and blue (B). Three primary color images are obtained after interpolations of images detected using a Bayer color filter array. Alternatively, three monochrome image sensors may be used to obtain three primary color images. No interpolation is needed. Each image sensor detects a primary color image without interpolation. Dichroic filters are typically used to selectively transmit or reflect a certain color, thus light having a specific color, either red, green, or blue, will arrive at and be detected by the monochrome image sensor. A monochrome image sensor is the same as a color image sensor without a color filter array covering light sensitive pixels.
In some applications, a fourth color or wavelength band may be required. For example, in a security camera, IR image detection is required for night vision. In another example, IR image detection is used for gesture recognition. In yet another example, an IR pixel is required for time-of-flight (TOF) detection. A TOF IR pixel may require specific circuit that is different from and more complex than regular circuits of RGB pixels. A TOF IR pixel is capable of detecting the distance from a target to the image sensor or the pixel. Consequently, implementing TOF IR pixels among RGB pixels in a single image sensor may not be straightforward. Accordingly, imaging systems having four image sensors are demanded.
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